The typical outboard propulsion unit used in industrial applications is a self-contained, deck-mounted device where power from a diesel engine is transmitted to the propeller through a system of drive shafts and right angle gears. Such units traditionally are offered in the 50-1000 horsepower range, and often are used for the propulsion of barges, small ships, and the like.
In the conventional prior device, the diesel engine flywheel is coupled to the outboard drive unit through a clutch, a reversing gear box, and an intermediate transmission that is designed to allow the outboard leg to be cocked or titled upward while maintaining power transmission to the propeller. This allows operation in shallow water conditions, and also allows the outboard unit to be elevated to the horizontal position for inspection and maintenance. The lower portion of the outboard drive assembly can be rotated through 360.degree. to allow full steering and maneuvering. The horizontal drive shaft at the top of the outboard drive assembly transmits power to the vertical drive shaft assembly through a 90.degree. spiral bevel gear. The vertical drive shaft assembly ma include several coupled sections. At the lower end of the vertical shaft assembly, power is transmitted to the propeller by another 90.degree. angle gear arrangement. The propeller shaft is provided with seals that are supposed to prevent water from entering the outboard drive assembly.
Steering typically is accomplished through use of a 90.degree. bevel gear, or a worm gear, that is used to rotate the lower section of the outboard drive assembly, which is supported by a suitable bearing. The steering drive shaft is powered, in the conventional example, by a bi-directional, low speed hydraulic motor mounted to the side of the vertical unit assembly and powered by a hydraulic pump that is driven from an auxiliary power take-off of the diesel engine. Cock-up, or tilting, has been accomplished by use of a hydraulic cylinder that is fed with pressurized oil from the same pump that is used for steering.
Although the above-described mechanical drive system has been widely used, it has a number of disadvantages and shortcomings in the areas of operation, reliability, longevity and maintenance requirements. Speed control of a mechanically drive, right angle geared drive shaft system is limited, because the engine can not be used effectively below its idling speed. Reversal of thrust can be accomplished only with a reversing gear, with lower response time due to higher moments of inertia, which is an extreme disadvantage in being able to cope with emergency situations. Since an auxiliary pump driven from an auxiliary power take-off on the engine is used for both steering and tilt-up, when the unit is operated at one-half speed, the auxiliary pump also runs at one-half speed. Therefore, the steering response is only one-half as fast as when the unit is operating at rated speed. By nature of its design, a mechanically drive unit is somewhat noisy in operation, especially during maneuvering when accelerations and decelerations cause back-lash in the reversing gear and the right angle drive gear transmissions. The various drive shafts vibrate in radial, axial and resultant directions, which promotes wear, as well as bearing and seal failures.
Still another disadvantage of a mechanically driven unit is that propeller speed control is accomplished by throttling the engine. Consequently, speed control during normal maneuvering involves numerous decelerations and accelerations of the engine. However, a diesel engine performs more reliably and efficiently, and has a longer service life, when operated at a constant speed at all times, which is not possible with mechanical drives. With respect to the steering gear arrangement typically used in mechanical drive systems, the engine torque is transmitted by a vertical drive shaft from the stationary to the steerable portion of the outboard drive assembly. Therefore, the steering gear must counteract the full propulsion torque to prevent the lower drive assembly from spinning around. Thus the steering gear is subjected to substantial stress loading during maneuvering and reversing, which can cause brinelling and early wear of gear and bearings.
In spite of best efforts to seal the propeller drive shaft against the entry of sea water, there appears to be no reliable way that moisture and silt can be kept out of the vertical drive assembly. Entry of moisture and silt will cause a corrosive and somewhat abrasive environment inside the unit, which, in mechanical systems, typically includes numerous machined and uncoated surfaces that are exposed. Components such as intermediate drive shafts, the gears and the bearings, normally are not made of corrosion resistant materials, so that they soon begin to corrode. This has the effect of shortening the service life of mechanically driven systems, and substantially increasing maintenance requirements.
Yet another shortcoming of mechanical drives of the type described arises in connection with accidental grounding of the lower unit. When this occurs, as it inevitably will, the vertical drive unit is subjected to bending moments far in excess of those that occur during normal operating conditions. This can result in temporary, and sometimes permanent, bending of components, which causes misalignment, seizure and occasional permanent damage to shafts, gears, or bearings. It also will be recognized that when a propeller impacts a solid object, the diesel engine drive can not stop instantaneously due to its large moment of inertia. In attempts to solve this problem, mechanical drive systems have included either a slip clutch near the engine, or a shear pin at the propeller. Slip clutches are not ideal, due to inability to accurately set break-away torque, so that propeller and shaft deceleration and clutch break-away can easily exceed stress levels where permanent damage to the propeller and/or transmission components will occur. Shear pins provide generally reliable protection, however the drive unit must be brought in for pin replacement and inspection prior to reuse.
The general object of the present invention is to provide a new and improved compass-type retractable outboard drive unit that obviates all or substantially all of the foregoing disadvantages of prior systems.
Another object of the present invention is to provide a new and improved thruster unit that includes a direct hydraulic drive to the propeller to increase overall unit reliability and longevity, with substantially reduced maintenance requirements.